Preheat the oven to 350°F, and line a 9X9 inch square pan with parchment paper. Leave some paper from the sides for lifting. Spray the paper with oil spray. Set it aside.
Melt the butter and chopped chocolate over a double boiler, and stir occasionally until it is smooth and lump-free. This may take up to 5-8 minutes.
½ cup Semi-sweet chocolate, 1¼ cup Butter
Add in both sugars, and mix until all is mixed in and smooth. Stir for another few minutes. Remove from the heat and set aside to cool down.
1½ cup Dark Brown sugar, ½ cup Granulated sugar
In the meantime, whisk together salt, flour, espresso powder, and cocoa powder.
1 teaspoon Espresso powder, ¾ cup All-purpose flour, 1 cup Cocoa powder, ¼ teaspoon Salt
Start adding the eggs, and vanilla extract to the butter mixture one at a time and mix well after each one. Be sure the butter mixture is cool enough. If it is too hot, the eggs will scramble.
4 Eggs, 1 tablespoon Vanilla extract
Pour the mixture into the prepared pan, and sprinkle the Reese smarties all over the batter and bake for 30-35 minutes.
½ cup Reese smarties
Let cool down in the pan for half hour, then lift the parchment papers from the side letting the brownie on the cooling rack cool down completely.